The goal of this project is to obtain information about the normal functioning of mammalian retinas. We study single neurons in the cat retina, investigate neural responses to photic stimuli using intracellular recording, and characterize neural morphology with intracellular injections of stains. A new, enzymatic microelectrode stain, horseradish peroxidase (HRP), produces a dense reaction product visible in the electron microscope. With this we have observed the synapses impinging on single, electrophysiologically studied retinal neurons. A new physiological class of amacrine cell, the AIII amacrine cell, has hyper-polarizing responses similar to those of horizontal cells. The HRP stain reveals that the synaptic input to these cells originates with rod bipolar and invaginating cone bipolar axon terminals and with one or more classes of other amacrine cells. We suspect the AIII amacrine may provide signals contributing to surround mechanisms operating in the inner plexiform layer (IPL). Patterned stimuli were constructed to examine the receptive field properties of the electroretinogram (ERG) and to relate these properties to those of single neurons. Results indicate that the ERG b-wave responses of the cat retina may also originate in part with surround mechanisms operating in the IPL.